From left, Reps. Paul Broun and P.J. Gingrey, Minister Derrick Grayson, Rep. Jack Kingston, Arthur Gardner, Karen Handel, and David Perdue stand at their podiums to deliver opening remarks during the Georgia Republican Party U.S. Senate Debate at the Columbia County Exhibition Center in Grovetown, Ga. on April 19. (Sara Caldwell/The Augusta Chronicle via AP)

This week's primaries in North Carolina, Indiana and Ohio marked the first in a steady stream of big decisions voters across the country will be making this spring and summer.

In some races, the tea party and GOP establishment will square off — and Republicans could help or hurt themselves in the general election depending on who wins. In others, Democrats will race to the left and even embrace the health-care law that figures to be politically toxic in many other campaigns.

Circle May 20 on your calendar. That will be a huge day for the Republican Party. In addition to a U.S. Senate primary in Georgia — one of the key states in the battle for the majority — the Senate's top Republican, Mitch McConnell, will be on the ballot in Kentucky. In Idaho's 2nd district, a major business vs. tea party battle will also be settled that day. Oh, and Democrats, there is something for you, too. The one-of-a-kind primary to decide who faces Gov. Tom Corbett (R) in Pennsylvania could go down to the wire.

Below is our rundown of the top 10 primaries. As a reminder, these races stand out because of deep rivalries or because the outcome will say something larger about the Democratic or Republican parties. Others matter because the outcome could increase or decrease a party’s odds of winning in the general election.

To the Line!

10. Kansas Republicans Senate primary: The GOP establishment is pretty confident about this one right now. Some residency issues seemed to open the door to challenger Milton Wolf, and Sen. Pat Roberts (R-Kan.) isn't exactly raising money like he should be. But the GOP landed a pretty serious attack on Wolf early on, pointing to graphic X-rays that Wolf joked about on his Facebook page. We're still waiting for a poll to show Wolf with a real shot here. (Date: Aug. 5)

9. Kentucky Republican Senate primary: It’s pretty much the same situation here, with the national GOP convinced that businessman Matt Bevin is basically sunk. Senate Minority Leader Mitch McConnell (R) has spent plenty of money to make sure of that – as much as $10 million already. From here, the question is whether the primary will have wounded McConnell at all for the general election against Secretary of State Alison Lundergan Grimes (D). (Date: May 20)

8. Louisiana open Senate primary: This is a nonpartisan primary that will take place on Election Day, so it’s a little different situation. It seems quite likely that Sen. Mary Landrieu (D-La.) will face a runoff; the real question is how close she is to 50 percent – the number she needs to hit either on Election Day or in a runoff — and how much the GOP can rally around one candidate before or after the November election. Rep. Bill Cassidy is the clear GOP establishment favorite. (Date: Nov. 4)

7. Iowa Republican Senate primary: State Sen. Joni Ernst (R) appears ascendant in this race and has locked down both establishment (Mitt Romney, for example) and tea party support (Sarah Palin). The big question is whether she can compete with businessman Mark Jacobs and his self-funding. Indications are that she has upped her fundraising pace significantly in recent weeks. The winner gets Rep. Bruce Braley (D), who polls show with a slight lead. If Republicans can get a good nominee and put this in play, they'll significantly up their chances of winning back the Senate. (Date: June 3)

6. Idaho's 2nd district Republican primary: Business vs. tea party. The Club for Growth vs. the Chamber of Commerce. Rep. Mike Simpson vs. attorney Bryan Smith. In the ongoing battle between competing wings of the Republican Party, this is a clear focal point. Smith's fundraising has not been great, and Simpson has Mitt Romney in his corner — an important endorsement in an area with a large Mormon population. This race is far from settled, but heading into the stretch run, Simpson doesn't appear to be in major trouble. (Date: May 20)

5. Mississippi's Republican Senate primary: State Sen. Chris McDaniel has been hamstrung by widespread attention paid to controversial comments he made over the years as a talk-radio host. But he still has the Club for Growth and Senate Conservatives Fund in his corner — which he will need, since Sen. Thad Cochran has a super PAC backing him. Of all senators facing primary challengers, Cochran still looks like he has the toughest fight on his hands. But everything is relative, and Cochran, like several of his colleagues, is looking in better shape than he was last year. (Date: June 3)

4. Hawaii's Democratic Senate primary: The primary is not until August, but this race has been tense since day one. In March, President Obama endorsed Sen. Brian Schatz, an early backer of his 2008 White House campaign. The endorsement means a lot in Hawaii, the state where the president grew up and is very popular. Rep. Colleen Hanabusa, the preferred choice of the late senator Daniel Inouye (D-Hawaii), is trying to unseat Schatz less than two years after he was appointed to replace Inouye. (Date: Aug. 9)

3. Pennsylvania's Democratic gubernatorial primary: Rep. Allyson Schwartz has been emphasizing her vote for Obamacare, a tactic we don't see every day at a time when the law looks to be an electoral albatross for so many others. But in this race to the left, it could be the key for Schwartz, who is trying to catch businessman Tom Wolf. She has less than two weeks left, and Wolf built a huge lead. In the general election, Corbett is probably the most vulnerable governor in the country. (Date: May 20)

2. Nebraska's Republican Senate primary: National tea party groups like the Club for Growth and leading conservative figures like Sen. Ted Cruz (R-Tex.) have gone all in for Midland University President Ben Sasse. If he loses, it will be an embarrassment for them. In the final week, wealthy bank executive Sid Dinsdale has become a threat as establishment favorite Shane Osborn has faded. Suddenly, the tea party has directed its attacks at Dinsdale ahead of Tuesday's primary rather than Osborn. Sasse's final ad, notably, is all about his support within Nebraska. (Date: May 13)

1. Georgia Republican Senate primary: The most crowded and arguably the most important GOP Senate primary of all, this one appears to finally be taking shape. Rep. Paul Broun (R), whom the GOP establishment feared most as its nominee, hasn't caught fire or raised much money. The frontrunners to make the runoff appear to be Rep. Jack Kingston and businessman David Perdue, with former secretary of state Karen Handel grabbing some key support but still underfunded. Rep. Phil Gingrey, meanwhile, is behind but has a bunch of money he can dump on this race in the final weeks. The big question will be how he uses it, and whether he targets anyone in particular with attack ads. Democrats contend this one's in play regardless of who wins. Republicans think it's off the table if anyone not named Broun or Gingrey wins. (Date: May 20. Likely runoff: Aug. 5)

Sean Sullivan has covered national politics for The Washington Post since 2012.

Aaron Blake covers national politics and writes regularly for The Fix.

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